[en] Steroid-16α-hydroxylase activities have been measured in normal and induced rat livers using four different substrates. The male/female activity ratio as well as the induction factor vary with the substrate indicating that steroid-16α-hydroxylase activity is a heterogenous enzyme. Experiments using specific inhibitors led to the conclusion that steroid-16α-hydroxylase is supported by at least two cytochrome P-450 forms, different from the cytochrome P-448.